2.4.3 Back propagation neural network (BPNN)
BPNN is a kind of multi-layer feedforward neural network used for
nonlinear functions, which trains the weights and thresholds for many
times. When the actual output does not match the observed, it enters the
error backpropagation stage: it updates the weights between the hidden
layer and the input layer, as well as the thresholds of the hidden layer
based on the error of the output values (Maier and Dandy, 2000) . In
this paper, the sigmoid (x )
function serves as the activation function for input-hidden layer and
hidden-output layer (see Figure 5 (c)), and the formula is as
followed: